PARIS, Jan 12 (Reuters) - The "Friends of Syria", an alliance of mainly Western and Gulf Arab countries who oppose Syrian President Bashar al-Assad, urged opposition groups on Sunday to attend this month's peace talks, saying there was no other route to a political solution.

With 10 days to go until the first direct talks between the opposition and President Bashar al-Assad's government - set for Jan. 22 in Switzerland and dubbed "Geneva 2" - Western backers have struggled to unify rebel groups.

The main political opposition body in exile, the Syrian National Coalition (SNC), has been plagued by internal bickering. It postponed a decision on whether to attend until next week after nearly a quarter of its 121 members threatened to resign following after the re-election of its Saudi-backed leader, Ahmad al-Jarba.

In a final statement, the 11 core Friends of Syria nations urged the SNC to attend the talks on the shores of Lake Geneva.

"We invite them to form, as soon as possible, a delegation of opposition forces to participate in the political process," the joint statement said.

"There is no other political solution," French Foreign Minister Laurent Fabius said. "There will be no political solution for Syria unless 'Geneva 2' meets."

Even SNC participation at Geneva 2 is no guarantee that there will be an interlocutor in the talks who can truly represent the armed rebellion.

Led from abroad, the SNC has limited influence over the fighters on the ground who are acceptable to the West, and none over the more radically Islamist rebels who have supplanted them on many fronts of the rebellion.

"TEST OF CREDIBILITY"

Speaking in Paris, U.S. Secretary of State John Kerry said attendance in Geneva was "a test of credibility".

"That's why I am confident they (the Coalition) will be there," he said.

Jarba, who attended the Paris meeting, did not say whether he believed the Coalition would go, but said he was reassured that the 11 nations had agreed that there could be no transition under Assad.

"We are all in agreement to say that Assad has no future in Syria," he said.

However, one of Geneva 2's main sponsors is Russia, which has shielded Assad from Western pressure on the U.N. Security Council and rejects the view that his departure is a necessary condition for a settlement.

Russian Foreign Minister Sergei Lavrov was due to hold meetings in Paris on Monday with Kerry and Lakhdar Brahimi, the U.N./Arab League envoy for Syria, to prepare for Geneva 2.

Lavrov met Jarba late on Sunday, Interfax news agency reported. "I understand that you are first of all concerned about the future of your homeland," Lavrov was quoted as saying. "We also care about the future of the Syrian people."

Syria plunged into civil war after an uprising against four decades of Assad family rule erupted in March 2011 and descended into an armed insurgency after the army cracked down on protests.

ASSAD GAINING GROUND

Assad's forces have recently been gaining ground against rebel fighters backed by the political opposition groups, and he faces little pressure to make concessions.

A Western diplomatic source said the SNC was right to debate whether it should attend Geneva as Damascus refuses to discuss Assad handing over powers.

"The attitude of the regime, which is sending a delegation to do everything but talk of a transition, endangers Geneva, as does the increase in violent bombardments in recent weeks," the source said.

"If Russia wants a serious Geneva, it must understand that there cannot be a disconnect between what is happening on the ground and the core diplomatic objective."

The source said the Paris talks had aimed to reassure Jarba that the 11 countries backed the opposition wholeheartedly, and that regional nations with links to fighters in Syria would encourage those fighters to support the SNC ahead of Geneva.

"The 11 agree that, whatever the constraints, provocations, refusals by the regime with regard to Geneva, we think it is important they go to Geneva. The illusion that the regime is willing to negotiate has to be broken."

Western powers have supported the opposition with rhetoric but have backed away from material aid as al Qaeda-linked groups take advantage of a power vacuum in rebel-held regions.

In their statement, the Friends of Syria, who include the United States, Saudi Arabia, Qatar and Turkey, said all armed groups had to respect democratic and pluralistic values, and recognise the political authority of the Coalition.

"Extremist groups play in the favour of the regime, damage the image of the democratic and legitimate Syrian opposition, and deprive them of local and international sympathy and support," the statement said. (Writing by John Irish; Editing by Kevin Liffey)

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